WASHINGTON, Feb 14 (Reuters) - One of the latest Obamacare pitches to get young adults to sign up for health insurance starts out with a mother's kitchen note reminding her grown son to enroll.

"Mom, you know I can't afford it," the young black man protests, as he sits down at a kitchen table next to a bespectacled woman with a laptop computer linked to the U.S. federal enrollment website, HealthCare.gov.

"But for the first time you can," she replies reassuringly. "You go to the HealthCare.gov website, compare quality plans and you could get help paying for it."

The government-sponsored television ad, which is airing on five national cable-TV channels, including ABC Family and TVLand, is part of an uphill battle to increase youth participation in President Barack Obama's signature domestic policy achievement. Youth participation in the program is a key factor in whether the program succeeds or fails in its first year.

U.S. government data released this week show the demographic of adults aged 18-34 rose only slightly by the end of January to 25 percent of total enrollment in private Obamacare plans.

That is well below the 38 percent that administration officials have talked about achieving to give insurers a strong mix of healthier members, whose premium payments help offset the cost of older, sicker policyholders.

Several top insurers have expressed unease about the mix of enrollment so far, and Republican opponents of the law see weak youth participation as the start of a downward spiral that will put the government on the hook for more spending to keep it alive.

The Obamacare marketing push has already been delayed repeatedly by the botched rollout of HealthCare.gov in October, requiring the administration to focus its efforts on fixing the website. Given those delays, experts say the Obamacare marketplaces could have trouble getting much above the 30 percent mark by the time enrollment ends on March 31.

Obamacare's advocates from the White House to federal and state agencies, health insurers, hospitals and grassroots nonprofit groups are launching the final push on Saturday, with what they call National Youth Enrollment Day.

"We're entering the sprint to the end," said Aaron Smith, executive director of Young Invincibles, a nonprofit group that is spearheading youth outreach.

The Centers for Medicare and Medicaid Services, the federal government's lead Obamacare agency, allocated $52 million for paid media in the first three months of 2014.

The campaign has already had help from singers Lady Gaga and John Legend and actress Olivia Wilde. Focus is now on sports celebrities including former professional basketball star Magic Johnson, who appears in a promotion alongside this week's Olympic Games. CMS will also run ads during the hugely popular college basketball playoffs known as March Madness.

Administration officials say young adults are only one audience for the effort. But the campaign is targeting 25 U.S. metropolitan areas that are home to 5 million uninsured young adults, according to a Reuters analysis of U.S. Census data.

"There's a variety of tactics," said Marlon Marshall, deputy director of the White House Office of Public Engagement. "All of those things add up to one big echo chamber. And that echo chamber is going to grow."

Health insurers are expected to fork out millions more to advertise their plans. Nonprofit groups Enroll America and the nonpartisan Ad Council have a $30 million TV campaign with singing pets.

The campaign competes with counter-messaging from Republicans and other Obamacare critics including Generation Opportunity. The young conservatives group sought to discourage enrollment last year with viral online videos picturing young men and women cornered in intimate medical settings by a creepy Uncle Sam puppet. The group says it is planning "a barrage of efforts," which it declined to detail.

CONCERN OVER PRICE HIKE

Obama's Patient Protection and Affordable Care Act barred longstanding insurance market practices that imposed sharply higher prices on people who were sick or older. That puts the onus on the program to spread insurers' risks between policyholders who need lots of medical attention and younger consumers who tend to be healthy and cheaper to insure.

The concern is that the fewer young adults sign up, the higher insurance costs may have to rise for 2015.

A Massachusetts health insurance marketplace, launched in 2007 under former Republican Governor Mitt Romney and widely seen as the prototype for the Obamacare exchanges, took nine months to breach the 30 percent mark, according to an analysis by Jonathan Gruber, an MIT professor and an architect of Obamacare and Massachusetts' earlier reforms.

Gruber and other experts say reaching about 30 percent participation is a more reasonable goal for Obamacare in 2015 than 2014 given its challenges, including bitter partisan opposition, scant funding, and a public that is largely misinformed about the law and the botched HealthCare.gov launch.

"It's going to take two or three years minimum to do what Massachusetts did," said Brian Haile, senior vice president for health policy at Jackson Hewitt Tax Service Inc., a tax preparation service that is promoting Obamacare enrollment.

Analysts at the nonpartisan Kaiser Family Foundation maintain that even if youth enrollment remains unchanged at 25 percent, it would add only 2.4 percent to 2015 premiums because the law compensates insurers for unexpected losses.

Obamacare's allies cite some distinct advantages. For one thing, their youth enrollment push is getting under way just as the Internal Revenue Service distributes hundreds of billions of dollars in tax refunds that could provide the cash for initial premium payments, experts say.

The relatively low income rates for young adults are also expected to make them some of the biggest recipients of government tax credits and other subsidies to buy coverage. Census data show 27 percent of America's 18.4 million adults aged 19 to 34 have no insurance, while mean earnings range as low as $15,800, versus $46,000 for the overall population.

"By far, the biggest sweet spot is younger people. They are absolutely eligible for the largest subsidies. Many of them though are individuals who haven't had insurance before," said Peter Lee, director of California's healthcare exchange.

Standing outside Madison Square Garden in New York on a bitter February night, Steven Abramson had little luck attracting young people with flyers that read "NEED Health Insurance?" to fans headed to watch the New York Knicks take on the Portland Trail Blazers.

The few women who did stop to take the pamphlets couldn't speak English.

"The generation of older people understand the need for health insurance, but with younger people, we have to convince them to sign up, even though it is the law," said Abramson, a volunteer with the Obama group Organizing for Action.